1. Field of the Invention
This invention relates to the field of grounding for static electricity build-up in dispensing systems.
2. Background of the Invention
As is readily apparent, a long-standing problem is to keep paper towels available in a dispenser and at the same time use up each roll as completely as possible to avoid paper waste. As part of this system, one ought to keep in mind the person who refills the towel dispenser. An optimal solution would make it as easy as possible and as “fool-proof” as possible to operate the towel refill system and have it operate in such a manner as the least amount of waste of paper towel occurs. This waste may take the form of “stub” rolls of paper towel not being used up.
Transfer devices are used on some roll towel dispensers as a means of reducing waste and decreasing operating costs. These transfer devices work in a variety of ways. The more efficient of these devices automatically begin feeding from a reserve roll once the initial roll is exhausted. These devices eliminate the waste caused by a maintenance person when replacing small rolls with fresh rolls in an effort to prevent the dispenser from running out of paper. These transfer devices, however, tend to be difficult to load and/or to operate. Consequently, these transfer devices are less frequently used, even though they are present.
The current transfer bar mechanisms tend to require the maintenance person to remove any unwanted core tube(s), remove the initial partial roll from the reserve position, and position the initial partial roll into the now vacant stub roll position. This procedure is relatively long and difficult, partly because the stub roll positions in these current paper towel dispensers tend to be cramped and difficult to get to.
In order to keep a roll available in the dispenser, it is necessary to provide for a refill before the roll is used up. This factor generally requires that a “refill” be done before the current paper towel roll is used up. If the person refilling the dispenser comes too late, the paper towel roll will be used up. If the refill occurs too soon, the amount of paper towel in the almost used-up roll, the “stub” roll, will be wasted unless there is a method and a mechanism for using up the stub roll even though the dispenser has been refilled. Another issue exists, as to the ease in which the new refill roll is added to the paper towel dispenser. The goal is to bring “on-stream” the new refill roll as the last of the stub roll towel is being used up. If it is a task easily done by the person replenishing the dispensers, then a higher probability exists that the stub roll paper towel will actually be used up and also that a refill roll be placed into service before the stub roll has entirely been used up. It would be extremely desirable to have a paper towel dispenser which tended to minimize paper wastage by operating in a nearly “fool proof” manner with respect to refilling and using up the stub roll.
As an enhancement and further development of a system for delivering paper towel to the end user in as cost effective manner and in a user-friendly manner as possible, an automatic means for dispensing the paper towel is desirable, making it unnecessary for a user to physically touch a knob or a lever.
It has long been known that the insertion of an object with a dielectric constant into a volume with an electrostatic field will tend to modify the properties which the electrostatic field sees. For example, sometimes it is noticed that placing one hand near some radios will change the tuning of that radio. In these cases, the property of the hand, a dielectric constant close to that of water, is enough to alter the net capacitance of a tuned circuit within the radio, where that circuit affects the tuning of the RF signal being demodulated by that radio. In 1973 Riechmann (U.S. Pat. No. 3,743,865) described a circuit which used two antenna structures to detect an intrusion in the effective space of the antennae. Frequency and amplitude of a relaxation oscillator were affected by affecting the value of its timing capacitor.
The capacity (C) is defined as the charge (Q) stored on separated conductors with a voltage (V) difference between the conductors:C=Q/V.
For two infinite conductive planes with a charge per unit area of σ, a separation of d, with a dielectric constant ε of the material between the infinite conductors, the capacitance of an area A is given by:C=εAσ/d
Thus, where part of the separating material has a dielectric constant ε1 and part of the material has the dielectric constant ε2, the net capacity is:C=ε1A1σ/d+ε2A2σ/d
The human body is about 70% water. The dielectric constant of water is 7.18×10−10 farads/meter compared to the dielectric constant of air (STP): 8.85×10−12 farads/meter. The dielectric constant of water is over 80 times the dielectric constant of air. For a hand thrust into one part of space between the capacitor plates, occupying, for example, a hundredth of a detection region between large, but finite parallel conducting plates, a desirable detection ability in terms of the change in capacity is about 10−4. About 10−2 is contributed by the difference in the dielectric constants and about 10−2 is contributed by the “area” difference.
Besides Riechmann (1973), other circuits have been used for, or could be used for proximity sensing.
An important aspect of a proximity detector circuit of this type is that it be inexpensive, reliable, and easy to manufacture. A circuit made of a few parts tends to help with reliability, cost and ease of manufacture. Another desirable characteristic for electronic circuits of this type is that they have a high degree of noise immunity, i.e., they work well in an environment where there may be electromagnetic noise and interference. Consequently a more noise-immune circuit will perform better and it will have acceptable performance in more areas of application.
The presence of static electric charges on a surface, which is in proximity to electronic systems, creates a vulnerability to the presence of such charges and fields. Various approaches to grounding the surfaces are used to provide a pathway for the static electric charges to leave that surface. Since static electric charges may build up from one or two kilovolts to 30 or more kilovolts in a paper-towel-dispensing machine, the deleterious effect on electronic components can be very real. An approach involves using an existing ground such as an AC ground “green wire” in a three-wire 110-volt system. The grounding is achieved by attaching to the ground wire or conduit. The grounding wire is ultimately connected to an earth ground. This approach is widely used in the past and is well known. However, many locations where a motorized paper towel dispenser might be located do not have an existing AC system with ground.
In cases where grounded receptacles are not present, a ground may be produced by driving a long metal rod, or rods, into the earth. Another method for grounding utilizes a cold water pipe, which enters and runs underground. Roberts (U.S. Pat. No. 4,885,428) shows a method of grounding which includes electrical grounding receptacles and insulated ground wire connected to a single grounding point, viz., a grounding rod sunk into the earth. This method of Roberts avoids grounding potential differences. Otherwise grounding each grounding receptacle to a separate grounding rod likely finds in-ground variation of potential. Soil conditions such as moisture content, electrolyte composition and metal content are factors that can cause these local variations in grounding potential. The cost and inconvenience of installing a grounding rod system may be prohibitive to support an installation of a motorized paper towel dispenser.
However, in many instances it may not be possible to have either of these approaches available. Therefore, a desirable grounding approach would be to ground to a local surface, termed a local ground, which may be a high impedance object, which is only remotely connected to an earth ground. In particular, dispensing paper towels, and other materials, can produce static electric build up charge during the dispensing cycle. In the past the static electricity build up, when it was produced on a lever crank or pulled-and-tear type systems paper towel dispensers, had little or no effect on the performance of the dispensing system. The most that might happen would be the user receiving a “static-electric shock.” Although unpleasant this static electric shock is not injurious to the person or to the towel dispenser.
Today, however, dispensing systems are often equipped with batteries. These batteries may operate a dispensing motor. However, in addition there may other electronic circuitry present, for example, a proximity sensing circuit might utilize low power CMOS integrated circuits. These CMOS integrated circuits are particularly vulnerable to static electric charge build up. It is desirable to protect these electronic from the static electric discharge.
In analyzing the static charge build up one may look at the charge separation occurring during a ripping operation of the towel or from the action of the paper on rollers or other items in the dispensing pathway.
A ground may be regarded as a sink of charge. This sink may be large as in the case of an actual earth ground. On the other hand, this grounding may relate to a relatively smaller sink of charge, a local ground. The sink of charge may be a wall or a floor or a part of such objects. The static charge build up may be in one sense regarded as a charge in a capacitor separated from a ground (as the second surface of the capacitor) by a high impedance material. The charge can't reach an earth ground as the wall material does not conduct electricity well.
There is, however, another mode of dispersing the charge on the surface. The isolated charges are of the same sign. The charges tend to repel each other. Therefore, the tendency is to spread out on the surface. Where the surface is completely dry and of a non-conductive material, then the actual conduction is very low. The motion of the charges, whether electrons or positive or negative ions, may be impeded by surface tension (Van der Waal) forces between the charges (electrons, negative ions or positive ions). Therefore, in the case where the surface is somewhat damp, even at a low 5% to 10% relative humidity, it is likely that various impurities are present in the water so as to form a weak, conducting electrolyte solution. At higher humidity this provides for an even more efficient way of dispersing the charges on the surface.